GCN Circular 31453

A. Rossi, F. Cusano, E. Palazzi, L. Amati (INAF-OAS Bologna), D. B. 
Malesani (Radboud U. Nijmegen and DAWN/NBI), S. Savaglio (UNICAL), on 
behalf of the CIBO collaboration, and D. A. Kann (HETH/IAA-CSIC) report:

We report the results of spectroscopic observations of two galaxies 
close to the position of the GRB (O'Connor et al. GCN #30934, Kann et 
al., GCN #30983) of the short GRB 210919A (Tohuvavohu et al., GCN 
#30846), obtained with the Multi-Object Double Spectrographs (MODS) 
instrument mounted on the 2x8.4-m LBT telescope (Mt. Graham, AZ, USA). 
Spectra were obtained at a mid-time of 12:35 UT on 2021-10-16, ~27.5 
days after the burst trigger, for a total on-source time of 5400 s, 
covering the wavelength range 3200-10000 AA.

The MODS slit inclination was adjusted to cover the positions of the two 
objects singled out by O'Connor et al. (GCN #30934), with offsets of 
~3.4 and 13" with respect to the candidate afterglow by Kann et al. 
(GCNs #30883 and #30884) respectively, and of the faint source at the 
eastern edge of the XRT error circle detected in the ESO-VLT/FORS2 
images (Kann et al., GCN #30983). This latter source was not detected in 
the MODS data. The spectra of the other two galaxies show several well 
detected emission lines which we interpret as [OII] 3727AA, H-beta, 
[OIII] 4959,5007AA and H-alpha at a common redshift for both sources of 
z=0.2411. The clear detection of these emission lines, and especially 
H-beta, indicates that these are two star forming galaxies.

At this redshift, the projected distances to the two galaxies at ~3.4" 
and ~13" from the afterglow corresponds to ~13 and ~50 kpc. The distance 
between the two galaxies corresponds to 63 kpc, similar to the distance 
between the Milky Way and the SMC. There is at least another galaxy (RA, 
DEC = 05:21:04.01, +01:18:49.23) within 0.5 arcmin with similar 
photometric redshifts (SDSS DR16), suggesting that the two galaxies are 
probably not isolated, but part of a larger group.

If GRB 210919A was at z = 0.2411, using standard cosmology (Planck 2016) 
and the analysis of the BAT spectra (Barthelmy et al., GCN #30863 and 
http://gcn.gsfc.nasa.gov/notices_s/1073893/BA), we find that it would be 
consistent with the population of short GRBs in the Ep,i - Eiso plane.

We acknowledge the excellent support from the LBTO and LBT-INAF staff, 
particularly D. Thompson, S. Allanson, and S. Paiano, in obtaining these 
observations.